________ she couldn't understand was ________ fewer and fewer students showed interest in her lessons.
A.What; whyB.That; whatC.What; becauseD.Why; that在线课程A
解析：
考查名词性从句．第一个空既要做understand的宾语又要引导一个主语从句，只能用what, that无此功能，因此排除B，C、D也不对，因为前后搭配不对．本句意思是“她不理解的是为什么越来越少的学生对她的课感兴趣”．“……的原因是……”用句型：the reason why…is that …．